Monday Quiz: Oscilloscopes and probes

Article By : Martin Rowe

Check your knowledge on making time-domain measurements.

Do you use an oscilloscope? Of course you do. Have you seen ringing and overshoot on digital signals? You know you have. Unless you deal with low-frequency signals such as audio, you have to contend with how an oscilloscope's passive probe can affect what you see on the screen. Even if you deal with low-speed digital signals, the rise and fall times can still be affected by loading from a probe.

Questions for this week's quiz come from the paper, High-Speed Time-Domain Measurements—Practical Tips for Improvement from Analog Devices.

1. If you want to accurately measure a signal at 100MHz, what's the minimum bandwidth your oscilloscope should have?

a. 100MHz
b. 200MHz
c. 300MHz
d. 500MHz
e. 1GHz

2. What is it about oscilloscope ground leads that makes them impractical for high-speed signal measurements?

a. Parasitic inductance.
b. Resistance in the clip lead.
c. Parasitic capacitance.
d. The loop of the ground lead acts like a receiving antenna.
e. The loop of the ground lead acts like a transmitting antenna.

3. What's the first thing you should do after connecting a probe be before making a measurement?

a. Calibrate the probe to ensure that the probe's internal LC constants are matched.
b. Calibrate the probe to ensure that the probe's internal RL constants are matched.
c. Calibrate the probe to ensure that the probe's internal RC constants are matched.
d. Go out for lunch to let the oscilloscope warm up.
e. Listen to The Measurement Blues for inspiration.

4. If your probes are not properly compensated, you can get ringing on rising and falling edges. What might cause you to not see ringing, even if it's actually there?

a. The oscilloscope's limited bandwidth hiding the high-frequency of the ringing.
b. The oscilloscope's sampling rate is too low.
c. The oscilloscope's timebase setting (time/div) is too long
d. The oscilloscope's timebase setting (time/div) is too short.
e. All of the above

5. Rise time is often defined as the time it takes for a signal to rise from 10% to 90% of full amplitude. You can assume that an oscilloscope and probe act like an RC low-pass filter. If the 10% point of the signal's rise time occurs at 0.1RC, then where is the 90% point?

a. 0.15RC
b. 0.2RC
c. 0.9RC
d. 1.2RC
e. 2.3RC
f. 3.0RC
g. 3.5RC
h. 5.0RC

Answers

1. If you want to accurately measure a signal at 100MHz, what's the minimum bandwidth your oscilloscope should have?

The correct answer is c, 300MHz. A signal loses about 30% of its amplitude at an oscilloscope's rated bandwidth.

2. What is it about oscilloscope ground leads that makes them impractical for the rise time high-speed signal measurements?

The correct answer is a, Parasitic inductance. The inductance in an oscilloscope's ground lead causes losses as frequency increases.

3. What's the first thing you should do after connecting a probe be before making a measurement?

The correct answer is c, calibrate the probe to ensure that the probe's internal RC constants are matched. Personally, I think the asnwer should be e, listening to The Measurement Blues for inspiration.

4. If your probes are not properly compensated, you can sget ringing on rising and falling edges. What might cause you to not see ringing, even if it's actually there?

The correct answer is e, all of the above. Well, the paper gives the answer as "This ringing and overshoot often go unnoticed due to limited bandwidth of a scope." But, in my search for wrong answers, I conclude that any of the answers could the cause.

5. Rise time is often defined as the time it takes for a signal to rise from 10% to 90% of full amplitude. We assume that an oscilloscope and probe act like an RC low-pass filter. If the 10% point of the signal's rise time occurs at 0.1RC, then where is the 90% point?

The correct answer is e, 2.3RC.

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